Semi-crystalline polymeric materials, such as polypropylene, are often employed in articles where it is necessary to subject the article to irradiation sterilization. The polymeric material without additives is subject, however, to degrading, embrittlement and discoloration during or subsequent to such irradiation. Many additives have been proposed for such semi-crystalline polymeric materials to inhibit the discoloration and degrading effects of the irradiation sterilization treatment. U.S. Pat. No. 4,100,185, for example, disclosed irradiation sterilization of semi-crystalline polymers wherein the semi-crystalline polymer has incorporated therein a non-crystalline mobilizing additive which increases the free volume of the polymer to thereby prevent embrittlement of the polymer during and subsequent to the irradiation.